1. Field of the Invention
The invention relates generally to optical recording and playback systems and more particularly to control circuitry in optical playback machines for deflection of the scanning light beam so that it accurately intercepts and follows the proper selected track segment in an optical record even though the record and the scanner optics are not precisely aligned.
2. Description of the Prior Art
In many optical recording systems, analog or digital data is recorded on an optical record in a data track which is segmented into arcs or raster lines across the record. Such records are illustrated in my co-pending application Ser. No. 556,780 filed and in my U.S. Pat. No. 3,501,586. Such records may be read by a playback machine such as that illustrated in my U.S. Pat. No. 4,090,031, which employs multiple sets of mirror-lens pairs as optical scanning elements to scan a light beam across a record. In each case, a track segment is read by one set of scanning elements and successive track segments are read by different sets of scanning elements as the scanner moves relatively to the record.
In principle, as each set of the scanning elements comes into position to read the next track segment, that segment is properly aligned with the scanning elements. In practice, there can be an error in the alignment of the light beam with the track segment to be scanned due to a tilted record or due to improperly aligned optical elements. In addition, for some purposes it is desirable to provide a random accessing capability for scanning track segments in an order other than that in which they are arranged on a record.
For example, in one kind of optical scanner, there are five sets of scanning elements on a scanning wheel, each set consisting of an objective lens and associated mirror. The sets are sequentially coupled to the light beam source one at a time by a distributor mirror in the form of a five sided polygon. The record may be a 4".times.5" plate or card with the record segments in the form of arcs across the width of the record. If the record is tilted slightly, the center of curvature of the arcs will not match that of the scanner. Consequently, the scanning light beam tends to follow a scanning path across the record which crosses the track segment rather than staying on such segment, as shown in FIG. 3.
In order to compensate for such tracking errors, the playback apparatus can include an optical tracker for locking the scanning light beam onto a track segment once the scanning path intersects such a segment. A variety of tracking techniques have been proposed in my U.S. Pat. No. 3,501,586 and No. 3,624,284, my co-pending application Ser. No. 645,806, filed Dec. 31, 1975, and U.S. Pat. No. 3,919,697 of Walker. However, none of the above references deals with the problem of initially positioning the scanning light beam properly on a track segment preparatory to scanning.
Furthermore, because operation of the optical tracker during the course of scanning a segment skews the scanning light beam from its uncorrected path as it scans the segment, use of such a tracker makes it more difficult to properly intercept the next track segment to be scanned. Referring to FIG. 3 it can be seen that, without an optical tracker, the light beam tends to traverse a scanning path shown in dashed lines which crosses the track segment. The optical tracker insures that once a track segment is crossed, the light beam will tend to follow it. However, in doing so, the tracker causes the light beam to deviate downward from the path it would otherwise follow by an amount Y, measured at the end of the scan. Then, as the next optical element begins scanning the record, its scanning path will be offset downward by the amount Y', thereby causing the scanner to skip the second track segment and incorrectly scan the third track segment. To avoid this problem, the record can be manually positioned to eliminate any error due to tilting, but it would be preferable not to have to make such manual adjustments. Therefore, there remains a need for a correction apparatus capable of operating together with an optical tracker, which will automatically insure that successive scans across an optical record start at the beginning of a correct next selected track segment.
FIG. 4 illustrates the effect of optical misalignment, assuming no record tilt, in a playback unit having four sets of optical elements. In FIG. 4, the first scanning path through the first optical element is properly aligned with a track segment, but the next three scanning paths are either above or below the track segments being scanned due to optical misalignment of successive optical elements. Previously, to avoid this problem, the optical axis of each objective lens was painstakingly adjusted to within 0.25 micron of a reference axis during assembly of the scanner portion of the playback device. It would be preferable to avoid having to manually adjust the alignment of each optical element to such close tolerances. Therefore, a need exists for alignment means in the playback unit for automatically adjusting the scanning light beam relative to the optical element to align it with each track segment as each optical element begins to scan such segment.
For some purposes, it is necessary to insure that the playback unit is scanning a particular identifiable track segment. Specifically, it would be desirable to identify a track segment that is being scanned and determine whether that segment is the correct track segment. If it is not the correct track segment, the playback device would be caused to reposition the scanning light beam to scan the correct track segment. Similarly, it would be desirable to be able to randomly access an identified track segment and to be able to move the scanning light beam which is presently scanning a first track segment the proper distance for intercepting a selected second track segment on the next scan.
Shoultes et al, U.S. Pat. No. 3,085,230, discloses the use of track addresses as a positive means of assuring that a read/write transducer is positioned on a desired track in a rotating magnetic disc record having a plurality of tracks with individual addresses. Shoultes employs digital addressing logic to operate a servo mechanism which moves the readwrite transducer to a radial position on the disc corresponding to a selected address. Each track has a gap followed by its address so that, as the disc rotates, the gap is detected. Detecting a gap triggers reading of the address and comparison with the selected stored address to produce an error signal. If there is no error, data is read from or written upon the track. If there is an error, the error signal triggers creation of a fictitious address which controls the servo mechanism in repositioning the transducer onto the correct track.
The Shoultes patent does not teach how track addressing could be employed in an optical playback system wherein the light detector (transducer) is stationary, while the scanner rotates, rather than the record which, instead, moves along a line transverse to the scanning path and data tracks. Nor does Shoultes teach a way to coordinate such track addressing functions with those of a track-follower designed to maintain registration of a track, while it is being scanned, with the detector. Thus, a need remains for a track intercept means whereby optical track segments can be identified and a scanning light beam directed to a selected track segment.